The present invention is particularly suited to briquetting organic material containing moisture, such as peat, lignite, brown coal, sub-bituminous coal and bituminous coal. The present invention is especially suited to briquetting sub-bituminous and bituminous coals and, for convenience and ease of description, the invention will be described with reference to the briquetting of sub-bituminous and bituminous coals. However, it will be understood that the invention is not limited to briquetting bituminous and sub-bituminous coals and that the invention may be used to briquette other organic or inorganic materials, and indeed any other materials that may or may not contain moisture.
Sub-bituminous coals have relatively high moisture levels (up to 35% moisture, by weight). Not only does this reduce the calorific value of the coal, it almost completely excludes sub-bituminous coals from export markets due to the poor economics associated with transporting the very high moisture levels associated with those coals. Removal of this moisture requires a thermal drying stage due to the chemically combined nature of most of the water in sub-bituminous coals. However, removal of the water causes further problems that prevent greater acceptability and use of such coals. In particular, once dried, the sub-bituminous coals suffer serious spontaneous combustion problems and considerable size degradation. Consequently, the drying process tends to increase the quantity of fines associated with the coals.
Bituminous coals are higher rank coals that have a higher calorific value and lower inherent moisture content than sub-bituminous coals. Although more amenable to transport than sub-bituminous coals, bituminous coals can suffer from production of excessive fines during mining, washing and handling operations. Such fines are difficult to use and represent a loss of product. Free surface moisture also causes difficulties with fine bituminous coals and can be as high as 25% by weight. This causes problems because it reduces the available energy content of the coal and can have a serious impact on the handling properties of the coal.
With many metallurgical and higher ranks steam coal, the grindability of the coal is such that the percentage of fine material in the total cleaned coal is sufficient to result in marketing difficulties even if the moisture content is reduced to an otherwise acceptable value. A possible solution to overcome this difficulty involves briquetting of the coal to form larger particles of the coal. However, the briquetting solution must provide for restructuring of the fines into a commercially attractive, handleable and durable form whilst not increasing production costs beyond the threshold of overall profitability.
Briquetting of coal has been carried out since the late 19th century. The traditional approach to coal briquetting involved mixing the coal particles with binders to bind the coal particles together into a larger mass. Binders that have been tried include organic binders such as coal-tar pitch, petroleum bitumen and asphalt, wood tar, synthetic and natural resins, starch, sulphite liquors, sugars and molasses, cellulose compounds, vegetable pulps, alginates, glue or gum, albumates, casein, peat, lignite and wood. Inorganic binders that have been used include cement, clay, lime, magnesia, gypsum, sodium or other alkali silicates. Compound binders comprising blends of two or more of the above have also been tried.
The criteria by which binders used in briquetting of coal have been judged include:                type of coal being briquetted        briquette strength and handling properties        moisture and weather resistance        combustion characteristics        effect on the physical properties of the coal        physical integrity of the briquette during combustion        toxicity of the binder or its de-composition and combustion products        cost of the binder        processing costs for the use of the binder, such as blending and curing.        
Experience with coal briquettes that incorporate binders has shown that a generally applicable coal binder that meets all of the above criteria has not yet been found. For example, pitch, bitumen and asphalt give adequate strength and moisture resistance but result in smoking combustion, toxicity problems, change to coking properties, high cost and high processing costs. Starch provides good strength and clean burning but is expensive and has poor weathering resistance. Sulphate liquor binders burn with little smoke but produce toxic sulphur dioxide emissions and have poor weathering properties. Sugars, particular molasses, have inferior moisture and weathering resistance and may suffer mould growth during storage. Cellulose-type binders typically suffer from low strength and rapid deterioration, with the briquettes tending to disintegrate in the early stages of combustion. It has been reported that the inorganic binders all suffer relatively low strengths, poor weathering resistance and high ash levels after combustion.
The necessity to add the binder component to the briquette also unavoidably increases the complexity and cost of briquette production.
In order to try to avoid some of the difficulties involved with producing briquettes using binders, various attempts have been made to produce binderless briquettes. For example, U.S. Pat. No. 2,937,080 in the name of Komarek et al describes a process for briquetting coal by feeding coal particles into a hot gas stream in a flash dryer. The coal particles are entrained by the gas stream and are heated by the gas stream to cause water to evaporate from the coal. The coal particles are transported by the gas stream to a cyclone separator, where the hot, dried coal particles are separated from the gas stream. The separated coal particles drop into a hopper and pass through the rolls of a briquetting apparatus to form the briquettes. This patent states that the temperature of the coal just before it reaches the briquetting machine should be just below the softening point, e.g. of nascent plasticity, of the coal for maximum efficiency and operation. The patent further stated that many coals can be compressed into suitable agglomerates within the temperature range of about 300 to 700° F. (about 149 to 371° C.).
U.S. Pat. No. 5,046,265 in the name of Kalb describes an integrated process for drying and briquetting sub-bituminous coals. In this process, coal is dried in a flash dryer under conditions that promote degradation of the coal particles. In particular, the coal particles are fed to a hot gas stream in a flash dryer and the thermal shock of the particles entering the hot gas stream causes degradation of the coal particles. The thus-treated coal particles are then separated from the gas stream by a series of cyclones. The separated gas is at least partially recycled to the flash dryer. The dried coal particles are fed to degasification/pre-compaction augers that compress the coal and remove gas therefrom before it is fed to the briquetting machine. In this regard, it will be appreciated that the dried coal particles have gas filling the interstitial spaces between the particles of coal and that the pre-compaction augers compress the coal and reduce the interstitial volume, thereby driving gas out of the coal. The gas liberated from the coal by the pre-compaction augers is treated by a small volume fan and dust collector. The pre-compacted coal is subsequently fed to a briquetting machine.
U.S. Pat. No. 5,046,265 specifies seven parameters as being critical to the briquetting process, these being:    1 The temperature of the material—water resistance and overall briquette quality were noted to improve significantly as temperature is increased from ambient to about 160 to 180° F. (71-82° C.). It has been also noted, however, that briquette quality began to decline if the material was heated to a temperature in excess of 220° F. (104° C.).    2 Feed size consistency.    3 Degasification and pre-compacting.    4 Compressive pressure.    5 Maintenance of an oxygen deficient atmosphere—The patent states it is known and demonstrated fact that sub-bituminous coal fines which have been thermally dried to moisture contents in the range of 10% or less (substantially below the inherent moisture content of undried sub-bituminous coal materials but above the preferred levels of 4-5% achievable by the present invention) are highly susceptible to rapid spontaneous ignition approaching spontaneous explosion when exposed to normal atmospheric concentrates of oxygen, even at ambient temperature conditions. For this reason, it is necessary that the entire portion of the process system containing the hot and dry fine coal must be maintained under inert (oxygen deficient) conditions. In this patent, the entire system is maintained under an inert gas environment via controlled “leakage” of inert gas from the dryer.    6 Cooling of the briquetted product.    7 Maintaining minimum quantities of combustion ash materials in the feed to the briquetting system.
The overall process system described in U.S. Pat. No. 5,046,265 is stated as being able to convert high inherent moisture (30-35%), low BTU value (8,200 to 8,800 BTUs per pound), sub-bituminous coals, and the like into a high BTU (11,000 to 11,500 BTU per pound), low moisture (approximately 5-8%) product which at the same time has acceptable handleability characteristics in the context of the current market place and user infrastructure system.
International patent application no. PCT/CA90/00056 (International Publication no. WO90/10052) describes a briquetting process for briquetting bituminous coals without the use of extraneous binders. This system uses high temperature, high pressure mould forming of the coal fines material in an apparatus incorporating a high pressure roll-type briquetting press. The system incorporates a positive pressure, controlled oxygen, gas re-circulation flash dryer and a direct briquette product to feed a heat exchange system for high system efficiency.